Inkjet recording apparatuses are widely utilized for image recoding apparatuses such as printers, facsimiles and copying machines. Such inkjet recording apparatuses are utilized for recording by delivering an ink from a recording head to a recording medium such as paper and has various advantages that a highly fine image can be recorded at higher speed, that the running cost is lower, that the inkjet recording apparatuses generate less noise, and that recording of color images is carried out easily using inks having various colors.
Such inks for inkjet recording typically contains water mainly, and also a colorant a wetting agent e.g. glycerin for preventing clogging of the ink, a penetrating agent for controlling the penetrability of the ink into the recording paper, and other additives.
Conventionally, papers exclusively utilized for purpose of inkjet printing process (hereinafter referring to as “inkjet paper”) have been developed, which have characteristics of improved absorption property, fixing ability of coloring component onto paper surface, and function to protect the coloring component so as to promote the infiltration and fixing of the liquid ink onto recoding media, in inkjet recording on the base of aqueous ink. However, the inkjet paper is produced by subjecting a made paper after the papermaking to various coating steps, thus the cost thereof is high and the inkjet paper comprises various processing agents, thus it has poor recycling properties in comparison with plain paper. Therefore it is desired that a satisfactory image quality can be obtained on the plain paper.
Since the plain paper has poorer ink absorption properties than those of the inkjet paper and cannot assist the ink function, like the inkjet paper, by using the plain paper, there are caused disadvantages such as (1) the cause of the feathering of the ink, (2) the cause of the bleeding of the ink, (3) the lowering of the ink density, (4) the lowering of the color developing properties of the ink, (5) the lowering of the water resistance of the ink, (6) the lowering of the light resistance of the ink, (7) the lowering of the gas resistance of the ink, (8) the lowering of the fixing properties of the ink, (9) the cause of a cock-ring curl and (O) the cause of the show-through of the ink.
Therefore, solving these disadvantages is an important task for the inkjet recording on the plain paper.
Recently, the dispersibility of the pigment has been improved and the particles of the pigment have been pulverized, then such a pigment has been frequently used for an inkjet ink. For example, the above-noted disadvantages (5) to (7) can be improved by using a pigment, such as a carbon black. However, the pigment has poorer density and poorer color-developing properties than those of a dye and is poor in the reliability, such as delivering stability; shelf stability for a long term and redispersibility. Therefore, when the pigment is used as a colorant, it becomes a task to improve the reliability of the recording ink.
Various inks for the inkjet recording using an emulsion of colored polymer particles, particularly of polyester or vinyl polymer particles are reported for solving the above-noted disadvantages (see Patent literatures 1 to 30). An ink comprising a colorant-involved resin dispersion in which a colorant is involved in a water-insoluble dispersible resin is disclosed in these patent literatures. Further, when a color organic pigment is used as a colorant, a conventional ink composition is more excellent in the image density and color reproducibility on the plain paper than a pigment ink using a water-soluble dispersing agent, however is poorer than a dye ink.
The compatibility between the improvement of the reliability and the improvement of the image quality has been studied as other treatments of the inkjet ink than the above-noted treatments. Many inkjet inks are so designed that the viscosity elevation of the ink is suppressed as much as possible for preventing the clogging of the head nozzle. For example, proposed is an ink in which by suppressing the viscosity of an ink which is concentrated by two times from an original ink within 10 times that of the original ink or by suppressing the particle diameter of an ink which is concentrated by two times from an original ink within 3 times that of the original ink, an agglomerated pigment can prevent the suppressing of the spread of the ink and can prevent the void of the ink (see Patent literature 31). However, by this proposed ink, an image having a high quality can be difficulty formed on the plain paper.
Also, proposed is an ink in which a liquid is a residual component of the ink after a volatile component of the ink is evaporated and the viscosity of the liquid is within 10 times that of the original ink (see Patent literature 32). However, this ink is a dye ink and while this ink has high reliability the image formed using this ink on the plain paper has a poor quality.
Further, proposed is an ink in which the viscosity of the ink in which water in the ink is evaporated at 60° C. is 600 times or less that of the original ink (see Patent literature 33). However, the proposed ink is also a dye ink and while the reliability of the ink and the durability of the image quality are balanced by incorporating a water-soluble polymer in the composition of the ink, this ink is problematic in water resistance.
It is also proposed that an ink having a high viscosity (e.g., 5 mPa·s to 15 mPa·s) is required for securing a high image quality (see Patent literature 34). In this proposal, it is described that for securing the reliability of the ink, it is advantageous that not only the initial evaporating rate of the ink is controlled, but also a specific compound as a viscosity-controlling agent for controlling the viscosity of the ink is incorporated in the composition of the ink, and there can be mentioned that this proposal is a solving method of the above-noted disadvantages (3) and (4) in the case of using a pigment. However in this proposal, there is no description with respect to the stability of the particle diameter of the used pigment and there is described only that the ink has reliability after the ink is left to stand for 24 hours. However, the described ink composition is poor in the reliability of the ink depending on the composition of the inkjet head from which the ink is delivered and on the size of the nozzle diameter, when the ink is left to stand further for a longer term.
As described above, an ink having a high viscosity is necessary to be used for securing a high printing quality at a high speed. However, the reliability of the ink having a high viscosity can be difficulty secured and such an ink is not made full use thereof.
Further, as a method for improving the image quality, the method for solving the above-noted disadvantages (1), (2) and (8) to (10) with respect to the plain paper has been attempted by elevating the drying rate of the ink without lowering an ink attaching amount.
Examples of the method for elevating the drying rate include (i) a method in which a volatile component is incorporated in the composition of the ink and (ii) a method in which the printer is equipped with a drying unit.
In the method (i), while the drying properties of the ink can be easily improved, the meniscus of the ink nozzle of the head is also easily dried and the viscosity of the ink is elevated, thus the stable delivery of the ink is reversely affected. Further, the printing environment is impaired, since a volatile component is an organic compound, thus the using of an aqueous ink loses the advantage thereof. With respect to the on-demand inkjet system using a pigment, there are various disadvantages from the viewpoint of the reliability of the ink.
The method (ii) becomes practicable by incorporating a heater in the image forming apparatus, however there is also such a disadvantage that the apparatus becomes complicated. As a specific method for heating the ink for drying the ink, such methods are studied as a method in which the film platen is heated and the recording medium is heated through a conduction of the heat (see Patent literatures 35 to 39), a method in which the ink is heated using a infrared light (see Patent literature 40) and a method in which such a fixing apparatus is used as a fixing apparatus using a heating roller (see Patent literatures 41 and 42).
However, in the method using the platen, a disadvantage is caused wherein the thermal conductivity to the recording medium is not uniform, in the method using a infrared light, a disadvantage is caused wherein a large amount of the electric power is necessary and in the method using the fixing apparatus, a disadvantage is caused wherein the apparatus becomes in a large scale. Further by heating a recording medium before or after the printing, not only the recording medium, but also the head surface is heated through a waste heat or a radiant heat Further, by heating the head surface, the drying of the meniscus is promoted and the delivering stability of the ink is lowered. It is also proposed that the affection of the heat can be suppressed by using the head surface as a heat reflecting surface (see Patent literature 43).
However, there is no description with respect to the ink in this proposal and from this fact, this proposal can be mentioned to have not satisfactory reliability.
Thus, the inkjet printer using an aqueous ink and equipped with a thermal fixing unit has a complicated mechanism and has disadvantage in the reliability of the ink However, many types of the inkjet printer are brought on the market for obtaining an advantage of improving the image quality during a high-speed printing or both-side printing. However, an on-demand inkjet printer using pigment inks as full-color inks which is equipped with a thermal fixing unit has yet various problems with respect to the reliability of the ink and the practical application thereof is delayed.
Thus, while using the process of the thermal fixing leads to solving the above-noted disadvantages (1), (2) and (8) to (10), the reliability of the ink remains as a problem. Further, while using a dye ink in combination with a pigment ink having a high viscosity leads to solving the above-noted disadvantages (3) to (7) with respect to the plain paper, a disadvantage is caused wherein the delivering stability of the ink is lowered in comparison with the case wherein the ink comprising only a dye is used and the reliability of the printer is extremely lowered.    Patent literature 1 Japanese Patent Application Laid-Open (JP-A) No. 2000-191972    Patent literature 2 JP-A No. 2000-191968    Patent literature 3 JP-A No. 2000-191967    Patent literature 4 JP-A No. 2000-53900    Patent literature 5 JP-A No. 2000-53899    Patent literature 6 JP-A No. 2000-53898    Patent literature 7 JP-A No. 2000-53897    Patent literature 8 JP-A No. 2000-44852    Patent literature 9 JP-A No. 2000-26775    Patent literature 10 JP-A No. 2000-7963    Patent literature 11 JP-A No. 2000-7962    Patent literature 12 JP-A No. 9-241565    Patent literature 13 JP-A No. 9-217030    Patent literature 14 JP-A No. 9-183932    Patent literature 15 JP-A No. 9-183931    Patent literature 16 JP-A No. 9-286939    Patent literature 17 JP-A No. 9-157565    Patent literature 18 JP-A No. 10-46082    Patent literature 19 JP-A No. 10-46092    Patent literature 20 JP-A No. 10-60327    Patent literature 21 JP-A No. 10-273610    Patent literature 22 JP-A No. 10-279851    Patent literature 23 JP-A No. 10-298462    Patent literature 24 JP-A No. 10-298467    Patent literature 25 JP-A No. 11-217528    Patent literature 26 JP-A No. 10-338829    Patent literature 27 JP-A No. 10-316918    Patent literature 28 JP-A No. 11-256083    Patent literature 29 JP-A No. 11-323226    Patent literature 30 Japanese Patent (JP-B) No. 3065950    Patent literature 31 JP-A No. 2002-337449    Patent literature 32 JP-A No. 2000-095983    Patent literature 33 JP-A No. 9-111166    Patent literature 34 JP-A No. 2001-262025    Patent literature 35 JP-A No. 55-69464    Patent literature 36 JP-A No. 55-84670    Patent literature 37 JP-A No. 58-12460    Patent literature 38 JP-A No. 62-130863    Patent literature 39 JP-A No. 62-130864    Patent literature 40 JP-A No. 57-120447    Patent literature 41 JP-A No. 3-169644    Patent literature 42 JP-A No. 6-184478    Patent literature 43 JP-A No. 11-123828